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Suppose the 10^19 photons emitted per se...

Suppose the `10^19` photons emitted per second from the 100 W lightbulb in were all focused onto a piece of black paper and absorbed. (a)Calculate the momentum of one photon and (b)estimate the force all these photons could exert on the paper.

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Each photon's momentum is obtained from, `p=n//lambda`, Next, each absobed photon's momentum changes from `p=h//lambda` to zero. We use Newton's second law, `F=Deltap//Deltat`, to get the force.
(a)Each photon has a momentum
`p=h/lambda`
`=(6.63xx10^(-34) J.s)/(500xx10^(-9) m)`
`=1.3xx10^(-27)` kg.m/s
(b)using newton's second law for `N=10^19` photons whose momentum changes from `h/lambda` to 0, we obtain
`F=(Deltap)/(Deltat)=((Nh)/lambda-0)/"1s"`
`=N h/lambdaapprox (10^19s^(-1))(10^(-27) kg.m//s)approx10^(-8)N`
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